Over the past few days, rates of vaccine-induced thrombotic thrombocytopenia (VITT) increased from 0.9 per 100,000 doses to 1.7 per 100,000 doses. This means that approximately 1 in 60,000 people could be affected by this rare but sometimes fatal side effect.
As of May 8, Ontario had seen eight cases of VITT. There were 654,123 doses of AstraZeneca administered in the province out of the 707,000 vaccines provided.
Health officials are currently reviewing the use of the vaccine for second doses and will provide guidance to those awaiting second doses of AstraZeneca in the coming weeks.
The vaccine is currently only available in pharmacies for adults aged 40 and over.
Health officials say their decision was also influenced by the high and reliable supply of Pfizer and Moderna vaccines.
Medical Officer of Health Dr David Williams said he looked at data from the UK which shows the risk of blood clots is much lower when AstraZeneca is used as a second dose.
Mixing doses of the COVID-19 vaccine is also under investigation.
Of the more than two million people vaccinated with AstraZeneca in Canada, 12 were diagnosed with vaccine-induced thrombotic thrombocytopenia (VITT), and three of them died.
Alberta became the first province to suspend administration of AstraZeneca as a first-dose vaccine because there were no new confirmed shipments of the vaccine.
Less than 200,000 doses remain of the 2.3 million already shipped, and only 1.65 million more are expected before the end of June.
With files from The Canadian Press